From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A957A17A2E1; Wed, 27 Aug 2025 09:30:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756287037; cv=none; b=q4Aj2CmyWL+Ys43gu77yEFLGWNY65PR1NGi3qpNKcQIHm8NkjV4z8T0LsKZoMTn3vu8fTqO9cLka4QINAIfxNU1nFuo1z63Kb6sUTIrWj+Vnw2e7pSNyNYLiCUx8GjFmTbw0SoklUz+I2ThfGccSCL0AYwo2xqNbXoEGBVUdtxA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756287037; c=relaxed/simple; bh=Ozxt1DUtnntTuQmp+vTRzOM+mQMEl1bfu+5vRxfUlis=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=iKTRmD2P39jZjzjCBcxR1Ui/58A3oURXUGuRzYkHlY2K9ufQXDrvPH8k2VIXsGTsYUI32g9ZFhYJWcJIWKzkK68CHN8lO6yHwn2LRH00qvVAvXHFjSoNd5ejyYkm3qQy532eixUlzii3YVHN3S6WCayNhfK7WfQwq7wMMkwupIg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=casper.srs.infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=oYlnTQjO; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=casper.srs.infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="oYlnTQjO" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=MIME-Version:Content-Type:References: In-Reply-To:Date:Cc:To:From:Subject:Message-ID:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=Ozxt1DUtnntTuQmp+vTRzOM+mQMEl1bfu+5vRxfUlis=; b=oYlnTQjO0soWYQDqmcP7oMq6BL /cm9scLW8WsCjYI0Wjmgh0O2rGvDHiyL1XctZar8616VGKA049JCIFQJbwakK6vexw9iVe8piXqda ZVm6Sn6bkuKEzkM8asP1zRKOQirWDtBn8rUAxpGq5BWC6a+vEeoVkubxBEhhjMet58b7ETwrFyQGs q7Rw/SDkj2y42+nBHLO/A69DdiPPIN+Ey7RUXiVSGAJjmR+5uyslclLRF6bb/Zo4EvysfO2r7ZogH Ulwey/eTmpYR2nRsxMzYSd9vVbW7mSdFeYAKalOvq2GjMZQymBddq+rg/eVqUVpqYK2yvMfLcK4EP WKNAecQQ==; Received: from 54-240-197-239.amazon.com ([54.240.197.239] helo=freeip.amazon.com) by casper.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1urCTs-00000002AGe-3kMX; Wed, 27 Aug 2025 09:30:13 +0000 Message-ID: Subject: Re: [PATCH v2 0/3] Support "generic" CPUID timing leaf as KVM guest and host From: David Woodhouse To: Sean Christopherson , Colin Percival Cc: Paolo Bonzini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Vitaly Kuznetsov , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, graf@amazon.de, Ajay Kaher , Alexey Makhalov Date: Wed, 27 Aug 2025 10:30:12 +0100 In-Reply-To: References: <20250816101308.2594298-1-dwmw2@infradead.org> <933dc95ead067cf1b362f7b8c3ce9a72e31658d2.camel@infradead.org> <6783241f1bfadad8429f66c82a2f8810a74285a0.camel@infradead.org> <01000198cf7ec03e-dfc78632-42ee-480b-8b51-3446fbb555d1-000000@email.amazonses.com> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-KxpmHU48OW8U1zXCQtDM" User-Agent: Evolution 3.52.3-0ubuntu1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org. See http://www.infradead.org/rpr.html --=-KxpmHU48OW8U1zXCQtDM Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Tue, 2025-08-26 at 12:30 -0700, Sean Christopherson wrote: > On Fri, Aug 22, 2025, Colin Percival wrote: > > On 8/21/25 14:10, David Woodhouse wrote: > > > On Thu, 2025-08-21 at 13:48 -0700, Sean Christopherson wrote: > > > > > I think I'm a lot happier with the explicit CPUID leaf exposed by= the > > > > > hypervisor. > > > >=20 > > > > Why?=C2=A0 If the hypervisor is ultimately the one defining the sta= te, why does it > > > > matter which CPUID leaf its in? > > > [...] > > >=20 > > > If you tell me that 0x15 is *never* wrong when seen by a KVM guest, a= nd > > > that it's OK to extend the hardware CPUID support up to 0x15 even on > > > older CPUs and there'll never be any adverse consequences from weird > > > assumptions in guest operating systems if we do the latter... well, f= or > > > a start, I won't believe you. And even if I do, I won't think it's > > > worth the risk. Just use a hypervisor leaf :) >=20 > But for CoCo VMs (TDX in particular), using a hypervisor leaf is objectiv= ely worse, > because the hypervisor leaf is emulated by the untrusted world, whereas C= PUID.0x15 > is emulated by the trusted world (TDX-Module). >=20 > If the issue is one of trust, what if we carve out a KVM_FEATURE_xxx bit = that > userspace can set to pinky swear it isn't broken? >=20 > > FreeBSD developer here.=C2=A0 I'm with David on this, we'll consult the= 0x15/0x16 > > CPUID leaves if we don't have anything better, but I'm not going to tru= st > > those nearly as much as the 0x40000010 leaf. > >=20 > > Also, the 0x40000010 leaf provides the lapic frequency, which AFAIK is = not > > exposed in any other way. >=20 > On Intel CPUs, CPUID.0x15 defines the APIC timer frequency: >=20 > =C2=A0 The APIC timer frequency will be the processor=E2=80=99s bus clock= or core crystal clock > =C2=A0 frequency (when TSC/core crystal clock ratio is enumerated in CPUI= D leaf 0x15) > =C2=A0 divided by the value specified in the divide configuration registe= r. >=20 > Thanks to TDX (again), that is also now KVM's ABI. And AMD's Secure TSC provides it in a GUEST_TSC_FREQ MSR, I believe. For the non-CoCo cases, I do think we'd need at least that 'I pinky swear that CPUID 0x15 is telling the truth' bit =E2=80=94 because right now= , on today's hypervisors, I believe it might not be correct. So a guest can't trust it without that bit. But I'm also concerned about the side-effects of advertising to guests that everything up to 0x15 is present, on older and AMD CPUs. And I just don't see the point in that 'pinky swear' bit, when there's an *existing* hypervisor leaf which just gives the information directly, which is implemented in QEMU and EC2, as well as various guests. --=-KxpmHU48OW8U1zXCQtDM Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCD9Aw ggSOMIIDdqADAgECAhAOmiw0ECVD4cWj5DqVrT9PMA0GCSqGSIb3DQEBCwUAMGUxCzAJBgNVBAYT AlVTMRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5jb20xJDAi BgNVBAMTG0RpZ2lDZXJ0IEFzc3VyZWQgSUQgUm9vdCBDQTAeFw0yNDAxMzAwMDAwMDBaFw0zMTEx MDkyMzU5NTlaMEExCzAJBgNVBAYTAkFVMRAwDgYDVQQKEwdWZXJva2V5MSAwHgYDVQQDExdWZXJv a2V5IFNlY3VyZSBFbWFpbCBHMjCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAMjvgLKj jfhCFqxYyRiW8g3cNFAvltDbK5AzcOaR7yVzVGadr4YcCVxjKrEJOgi7WEOH8rUgCNB5cTD8N/Et GfZI+LGqSv0YtNa54T9D1AWJy08ZKkWvfGGIXN9UFAPMJ6OLLH/UUEgFa+7KlrEvMUupDFGnnR06 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